At the time of A Realm Reborn initial launch, it was the most challenging content available. Initially, players could only enter a pre-made party into a turn the leader had not yet cleared, and if any member was further than the leader, entry was denied, and each turn could only be done in sequence. With Patch 2.2 and the début of the Second Coil of Bahamut, the Binding Coil can be accessed through Duty Finder and grants a fixed 20% Echo bonus to all parties.

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In the aftermath of the Battle of Carteneau, a fragment of Dalamud landed on an area on which a force from the Garlean Empire build the fortress Castrum Occidens on. After the declaration of the Seventh Astral Era, Alisaie Leveilleur recruits the adventurer to aid her in exploring the Binding Coil of Bahamut to discover the fate of Bahamut and her grandfather Louisoix, and to ensure Garlean forces do not obtain the Allagan technology Dalamud's still active defensive systems possess.

As they travel past the Imperial Exploratory Outpost, they discover the severed, colossal wing of Bahamut embedded in corrupted crystal formations. They descend into a buried Ragnarok-class starship that jettisoned from Dalamud, using its drive cylinder as a makeshift elevator. At the bottom of a chasm they find the right hand of Bahamut, along with a fearsome dragon enslaved by the Allagans.

Situated upon Vylbrand, Castrum Occidens is more akin to a research facility than a fortress. Even before its first pillar was driven into the ground, Legatus Gaius van Baelsar had ordered his people to probe the fragment of Dalamud which fell at that precise location. The impact resulted in the creation of vast caverns pervaded by corrupted crystals. Though unknown dangers lie in wait within, you must brave them all if you are to learn the truth of the Calamity.

—In-game description

Requires average item level 70 or higher

Entering the passage leading inward, the party must battle a mid-boss, an ADS (Automated Defense System) of Dalamud. Throughout the fight, it will use High Voltage to paralyze the party, making it recommended to Silence the attack. During the battle, smaller Attack Drones will appear from behind and should be dealt with by an off-tank.

Once the ADS is eliminated, progressing further leads to an open area. Several powerful Dark Matter creatures are found in this section. At two points, a Dark Matter Golem controls a barrier preventing further progress. These golems can use a dangerous Obliterate attack, which can be lethal to weaker party members.

A cutscene appears at the final section, where an enormous snake called Caduceus attacks. The Caduceus progressively gains an attack boost, but this can countered by bringing Dark Matter Slimes to it. At 60% health, it splits into two snakes, each having half the remaining health. Both must be slain quickly to avoid being overwhelmed by the remaining snake.

At the depth of 912 yalms, you find your path blocked by Dalamud's internal defenses. Born of advanced Allagan science, these spherical constructs are still fully functional even after eras in existence, and will seek to annihilate all intruders or be destroyed in attempt.

—In-game description

Requires average item level 73 or higher

In this ziggurat-like turn, the party must eliminate defense nodes while making their way to the ADS. Defeating a certain node will prevent the ADS from using an ability, but will grant it a passive buff that enhances certain traits. The goal is to carve a path to the ADS and defeat nodes based on party setup (left path for melee heavy groups, right path for magic heavy groups). The Quarantine Node in the southernmost room can be slain to disable the Allagan Rot, but doing so grants ADS a haste effect.

To complete this turn, the ADS must be defeated in the center. Unless the Quarantine Node was eliminated, it will use Allagan Rot on a party member. This effect has an 18 second countdown, and must be passed by standing next to a party member to reset it before it severely damages the party with an explosion. An "infected" player cannot receive it again for 40 seconds after transfer, so the party must coordinate when to pass it.

The ADS will slowly prepare an "enrage" mode after the instance starts, where upon it will repeatedly hit the party with Object199. As this disables all other abilities (including Rot), most players opt to let its time elapse and simply heal through Object199 barrage while performing an all-out attack.

At the depth of 1,233 yalms, your party comes upon the fragment of Dalamud the Garleans had sought. Its true identity is the Ragnarok, a colossal starship which fell away from the lesser moon moments before Bahamut burst free of his prison over the Carteneau Flats. To reach your destination, you must venture through this relic of ancient Allag. Stay ever on your guard, for every shadow may conceal dangers.

—In-game description

Requires average item level 70 or higher

Unlike other turns, there is no boss here. It is simply a gauntlet of jump pads that descend the hull of the starship. Throughout this turn several Dark Matter creatures are fought (or avoided) while heading to the exit.

At 1,524 yalms below the surface, employing the Ragnarok's drive cylinder as a makeshift lift, your party proceeds toward the main bridge. Yet you must work for every yalm you gain, for Dalamud's internal defenses will not allow you to pass unmolested.

—In-game description

Requires average item level 77 or higher

This turn is an elevator encounter. Rather than a singular boss, there are waves of clockwork creatures that must be fought. The most dangerous are the Clockwork Dreadnoughts, which hit hard and have more health than others. The waves consist of the following:

Having overcome the seemingly endless waves of hostile constructs, you and your stalwart companions continue the descent into an unknown abyss. At the depth of 1,910 yalms, you are assailed by a winged terror: an ancient dragon brought to heel by the Allagans and made to do their bidding. The trials grow ever more arduous, but you are resolved to press on for the sake of the truth.

—In-game description

Requires average item level 82 or higher

The final and most difficult turn, Turn 5, features the first superboss in A Realm Reborn: Twintania. Unlike the previous turns (or most boss fights for that matter), the dragon must be fought through challenging set of phases. Even the most well-geared players can be overwhelmed by some of her attacks.

The Binding Coil of Bahamut features musical themes based on "Answers;" the main theme of the dungeon called "Primal Timbre" shifts between a quiet, atmospheric version or a more dramatic version with heavier instrumentation based on whether the party is in combat with monsters, and the dungeon's "boss" theme also uses strains from the song in certain sections, though it is less obviously based on it. The exception is Twintania, which plays "Thunderer" as its battle theme instead.